This invention relates to a drive circuit for an average-responding automatic iris for a television camera for providing a peak-responsive control characteristic.
In the surveillance and industrial camera field, an extremely wide range of ambient light conditions may be encountered by a camera in the course of operation. For example, a camera which is used for surveillance may be subjected to high levels of illumination during daytime conditions and extremely low levels of illumination on overcast nights. The imagers used for such cameras include devices such as vidicons and solid state sensors known generally as CCD sensors. Such imagers have a relatively limited dynamic range. An automatic iris control is described in U.S. Pat. No. 2,885,471 issued in the name of G. King on May 5, 1959. Such an iris is coupled to the optical path of the imager and is connected in a degenerative feedback loop which maintains the output of the imager within a relatively constant range. The King arrangement generates a control signal in response to the peak-to-peak value of the video signal. Thus, when the sensing device views a scene containing highlights, the control circuit senses the highlights and closes the iris so the highlight region has adequate contrast.
In the surveillance and industrial camera market, it is desirable that a camera be adaptable for use with standard optical systems. Such optical systems may include zoom type telephoto lenses, light amplification devices and the like. Among the optical devices currently enjoying widespread use is the Cosmicar auto-iris "ES" series lenses developed by Cosmicar Optical Company Limited of 424, Higashi-Oizumi, Nerima-Ku, Tokyo, Japan. This automatic iris is desirable because of its quality optics and wide range of control. However, the ES lenses include internal circuitry which responds to the average value of the video signal produced by the camera. Such average-responsive automatic irises respond to general scene illumination but ignore highlight areas. This may be disadvantageous for particular applications in which the highlight areas include desired information. Heretofore, it has not been possible to use ES lenses in applications in which general scene illumination is low by comparison with the highlight areas, and it is desired to have contrast in the highlight areas.